The present invention relates to controlling, i.e. regulating, a wind turbine.
Procedures are already known relating to wind turbines such as that shown in FIG. 10, comprising a rotor hub 1 fitted with at least one blade 2 which is configured at an adjustable rotor-blade angle relative to said rotor. Said known procedures regulate the rotor's speed of rotation, hereafter termed rotor speed, while adjusting the blade angle within a predetermined range of wind speeds to set a predetermined power output.
FIG. 10 further illustrates known elements used n wind turbines, including an actuator 3 for changing rotor blade angles and a sensor 4 for determining the rotor blade angle. A rotor shaft 6 transfers rotational motion to a transmission 7 and further to a generator 8. The generator 8 includes a sensor 9 attached thereto for sensing power output. A signal line 14 transfers information from the generator sensor 9 to a PID controller 10. An anemometer 11 senses wind speed and provides this information to the PID controller 10 via a signal line 16. Blade angle position is transferred from the blade angle sensor 4 to the PID controller 10 via a signal line 12. The PID controller 10 can change the blade angle via a control line 13. The PID controller can change torque of the generator via another control line 15. The rotor hub also includes a connection 5 for another rotor blade.
Wind turbine regulation is didactically discussed below by means of the so-called power-output/rotor-speed characteristic curve shown as a function of wind speed in FIG. 1.
Conventional power-output and rotor speed characteristic curves presently are predominantly applicable to wind turbines and comprise two ranges.
The first range of the power-output or rotor speed characteristic curve is the lower partial-load range LR. This range begins at the so-called wind cut-in speed vK and terminates at the nominal speed vN. The cut-in speed is the wind speed at which the wind turbine delivers effective power. As regards wind speeds below the cut-in speed vK, the rotor power output merely covers power losses of the drive train and intrinsic needs.
In the lower partial-load range, the power output or the rotor speed increases with wind speed up to where the nominal wind speed vN is reached. Within this range, the blade angle is kept substantially constant and the rotor speed is regulated, i.e. controlled by the rotor-speed dependent torque. The power output depends on the wind-power collected by the rotor. The rotor-speed characteristic curve within the lower range comprises two segments, a and b, the segment “a” being level along the lower rotor-speed limit Ωu. Within the second segment “b”, the rotor speed rises linearly with the wind speed (operation at optimal tip speed ratio) until the upper rotor-speed limit in the form of the nominal rotor speed has been reached. The nominal rotor speed may be reached at the nominal wind speed, however said nominal rotor speed, most of the time, will be reached earlier.
The nominal wind speed vN is the wind speed at which the wind turbine's output for the first time corresponds to its so-called nominal output.
The second range of the power-output or the rotor-speed characteristic line is the so-called nominal power output range NR. This nominal power output range begins at the nominal wind speed vN and terminates at the shutdown speed vS; the regulation of rotor speed by adjusting the blade angle to set a nominal power output takes place within this nominal power output range. The shutdown wind speed vS is the maximum wind speed at which the wind turbine may be operating when delivering power. Ordinary operation of a wind turbine at which the nominal power output is regulated by adjusting the rotor blade is impossible at wind speeds higher than said shutdown speed because further exposure to mechanical loading would damage, even destroy the wind turbine. In the present state of the art, wind turbines are shut down when the shutdown wind speed is being reached.
In general, the wind turbine output power is fed into public electricity grids. However, abruptly shutting down wind turbines at wind shutoff speeds and especially so where the area contains many such wind turbines, may entail voltage or frequency dips in the public electricity grid. Desirably therefore, wind turbine shutdown should not be abrupt but instead in the form of a slowly reduced power output.
Moreover, besides grid compatibility, slow power reduction also allows continued wind turbine operation in spite of the shutdown speed having been exceeded, without thereby the wind turbine being damaged by increasing wind speeds and their attending increasing stresses.
One procedure whereby the output power and the operational rotor speeds can be regulated at and above the wind shutoff speeds as a function of wind speed illustratively is known from the European patent document EP 0 847 496 B1.
FIG. 2 illustrates an output power or a rotor-speed characteristic curve of the above known procedure, said curves comprising a third range segment as compared with the curves discussed earlier above, namely the upper partial-load range UR. This third segment begins when reaching a limit speed vLim substantially being the heretofore shutdown wind speed.
It is known from the German patent document DE 198 44 258 A1 to reduce the power output at a predetermined wind speed. However this reduction in power output is carried out at less than the shutdown wind speed.
Both procedures, stated above, of the state of the art share the feature of regulating the power output in the upper partial-load range upon reaching a predetermined limit speed as a function of the measured, increasing wind speed. However the practice of converting the wind turbine regulation in the upper partial-load range as a function of measured wind speed is exceedingly problematical.
Operating a wind turbine in an upper partial-load range involves subjecting the wind turbine to high loads caused by high wind speeds. One reason is that the air flow is not steady-state but turbulent. Turbulence implies that the speed at which the wind impinges the rotor surface is not uniform on said surface, instead the incident wind power is distributed unevenly. In other words, exceedingly high wind forces may be applied at one or more rotor zones. At the same time one or more other rotor zones may be little wind-loaded or not at all, whereby at high turbulence the wind turbine may be subjected to very high changing loads and be damaged by them at once or in the long term. At high wind speeds, the applied changing loads will be especially high.
Accordingly, the just above described changing loads due to turbulence should also be taken into account in power-output or rotor-speed regulation in the upper partial-load range.
As a result, as regards the known procedures, it is mandatory that the measured wind speed basic to their regulation shall properly reflect the actual wind conditions. Heretofore, however, no practical procedure has been available to reliably determine such basic wind speed conditions.
It is known to measure wind speed using a nacelle anemometer. This procedure however incurs a well known lack of accuracy, the wind turbine rotor interferes much with the measurement. Rotor effects are felt at least one rotor diameter in front of and three rotor diameters behind the rotor. Accordingly, clean-cut measurement is impossible. Moreover this procedure allows only measuring wind conditions in one spot of the wind zone. This known procedure is unable to transduce the turbulent wind speeds across the entire rotor.
Several publications, for instance WO 2004077068 A1 or German patent document 101 37 272 A1 disclose 3D measurements of wind conditions in front of the wind turbine using so-called LIDAR or SODAR systems. Such measurements are stated as being applicable in particular to wind farms.
To-date, for lack of sufficient testing, said systems have been put to practice only in isolated cases. Moreover, these LIDAR and SODAR systems are very expensive and it appears that economically not every wind turbine can afford to have one, hence these systems appear impractical when actual wind conditions must be ascertained.
Also the European patent document EP 1 230 479 B1 discloses a procedure using sensors configured in the rotor blades to detect their mechanical loads, the test values also being available for wind turbine regulation for instance. FIG. 2 of said document shows a schematic of power output as a function of wind speed and FIG. 3 shows a schematic of the tested blade loads also as a function of wind speed.
As shown by FIGS. 2 and 3 of said European patent document 1 230 479, the blade load is flat in the wind speed range where, according to FIG. 2, the power reduction should have begun. Therefore the mechanical loading of the rotor blades on the average remain substantially constant due to adjustment of the blade angle and the related reduction of the blades' wind-loaded surfaces. Consequently, blade loading may not be used as a drive parameter to regulate the power-output or rotor-speed characteristic curve in the upper partial-load range because such regulation operates only upon a clear change of the mean blade load, this condition being absent from FIG. 3. Indeed short-term load peaks may be used to regulate the rotor for instance to preclude collision between the rotor blade tips and the tower. However wind turbine or rotor-speed regulation by means of load peaks is not advantageous because grid compatibility and large mechanical rotor inertia of the wind turbine entail respectively slow changes in power output and in rotor speed enduring well after said peaks have decayed.
Accordingly, the procedure recited just above merely offers a way of detecting the mechanical loading of the rotor blades in said upper partial-load range and to take into account said detected data in the regulation of this range. However, the patent document cited just above does not disclose using such detected load values, and accordingly regulation in the upper partial-load range discussed in the European patent document EP 1 230 479 can only be carried out as a function of wind speed.
Summarizing the above cited procedures of the state of the art, it has been known heretofore to regulate the power-output or the rotor speed in the upper partial-load range as a function of wind speed. However, absent high costs, heretofore it has been impossible to detect a wind speed that would represent the actual wind conditions and therefore said above discussed previous procedures incur the substantive drawback that they make practical implementation difficult.
The known state of the art has failed so far to discover a method, in particular applicable to a multi-megawatt wind turbine, allowing operation in a power-output reducing manner in a wind speed range illustrative between 25 and 35 m/s or 25 and 40 m/s, and reliably excluding load increases on the wind turbine(s). The regulation algorithms of the known state of the art moreover are highly susceptible to interference especially for high wind speeds where fluctuations in such wind speeds are related to large fluctuations of incident wind power. For that reason the above cited procedures indeed have hardly been used practically.
Accordingly, the reduction in power output or in rotor speed should not be based on measured wind speed, but instead on an input variable which on one hand is easier to detect physically and by control technology and which on the other hand represents a better wind turbine stress signal.